FIG. 74 is a schematic front view for illustrating a spot welding process for joining high resistance workpieces 1, 2, which are made of a so-called high tensile strength steel and have a large thickness to exhibit a high electric resistance. The two high resistance workpieces 1, 2 are stacked to form a stacked body 3. The stacked body 3 is gripped and pressed between a first welding tip 4 and a second welding tip 5. When the first welding tip 4 and the second welding tip 5 are energized, a portion is heated to form a melted portion 6 in the vicinity of the contact surface between the high resistance workpieces 1, 2. Then, the melted portion 6 is solidified to generate a solid phase, which is referred to as a nugget.
Since the high resistance workpieces 1, 2 have the high electric resistance, a large amount of Joule heating is generated in the vicinity of the contact surface during the energization, so that the melted portion 6 grows larger as shown in FIG. 75 in a relatively short time. Therefore, the melted portion 6 is liable to be scattered (spatter generation is liable to be caused). Thus, in the spot welding process for joining the high resistance workpieces 1, 2, it is necessary to highly accurately control a welding current in view of preventing the spatter generation. However, such control cannot be achieved easily. This problem is caused even in the case of joining a thinner high tensile strength steel workpiece.
In the case of joining three or more workpieces, the workpieces may contain different materials and may have different thicknesses. For example, as shown in FIG. 76, an outermost workpiece (a low resistance workpiece 7) may have the smallest thickness. Incidentally, in FIG. 76, the low resistance workpiece 7 is made of a mild steel, exhibits a low electric resistance, and is stacked on the high resistance workpieces 1, 2 shown in FIGS. 74 and 75 to form a stacked body 8.
In the process of spot welding the stacked body 8, a larger amount of Joule heating is generated in the vicinity of the contact surface between the high resistance workpieces 1, 2 than in the vicinity of the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2. This is because a higher contact resistance is generated in the vicinity of the contact surface between the high resistance workpieces 1, 2.
Therefore, in the stacked body 8, a melted portion 9 is developed first in the vicinity of the contact surface between the high resistance workpieces 1, 2. As shown in FIG. 77, the melted portion 9 may grow larger before another melted portion is developed in the vicinity of the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2. When the energization is continued to form the other melted portion in the vicinity of the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2, the spatter generation may be caused in the vicinity of the contact surface between the high resistance workpieces 1, 2.
However, if the energization is stopped, the melted portion and hence the nugget are not grown to a sufficiently large size in the vicinity of the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2. Accordingly, a desired bonding strength is hardly achieved between the low resistance workpiece 7 and the high resistance workpiece 2.
This problem may occur also with an indirect feeding type welding apparatus.
FIG. 78 is a schematic side view of a stacked body 14 of three the metallic plates 11, 12, 13 gripped by an indirect feeding type welding apparatus 15. The indirect feeding type welding apparatus 15 has a first welding gun (not shown) for supplying a welding current and a second welding gun 16 for welding the stacked body 14. The welding current is transferred from the first welding gun through an external feed terminal 17 to the second welding gun 16. Such a structure of the indirect feeding type welding apparatus 15 is known from Japanese Laid-Open Patent Publication No. 07-136771, Japanese Laid-Open Utility Model Publication No. 59-010984, etc.
Specifically, the first welding gun has a positively (+) polarized upper electrode 18 and a negatively (−) polarized lower electrode 19. The second welding gun 16 has an upper tip 20 corresponding to the first welding tip and a lower tip 21 corresponding to the second welding tip. The external feed terminal 17 is prepared by interposing an insulator 23 between conductive terminals 22a, 22b. The upper electrode 18 and the upper tip 20 are electrically connected by the conductive terminal 22a and a lead 24, and the lower electrode 19 and the lower tip 21 are electrically connected by the conductive terminal 22b and a lead 25.
In the welding process, the stacked body 14 is gripped between the upper tip 20 and the lower tip 21 of the second welding gun 16. The welding current flows through the stacked body 14 from the upper tip 20 to the lower tip 21 in the thickness direction. A portion is heated to form a melted portion in the vicinity of each of the contact surface between the metallic plates 11, 12 and the contact surface between the metallic plates 12, 13. Then, the melted portions are solidified to generate solid-phase nuggets, whereby the metallic plates 11, 12 are connected and the metallic plates 12, 13 are connected to each other.
In a case where the metallic plates 11, 12 are the high resistance workpieces, which are made of a high tensile strength steel, have a large thickness, and exhibit a high electric resistance, and the metallic plate 13 is the low resistance workpiece, which is made of a mild steel and exhibits a low electric resistance, a larger amount of Joule heating is generated in the vicinity of the contact surface between the metallic plates 11, 12 (the high resistance workpieces) than in the vicinity of the contact surface between the metallic plates 12, 13 (the low resistance workpiece and the high resistance workpiece). This is because a higher contact resistance is generated in the vicinity of the contact surface between the metallic plates 11, 12.
Therefore, in the stacked body 14, as shown in FIG. 79, a melted portion 26 is developed first in the vicinity of the contact surface between the metallic plates 11, 12. The melted portion 26 may grow larger before another melted portion is developed in the vicinity of the contact surface between the metallic plates 12, 13. When the energization is continued to form the other melted portion in the vicinity of the contact surface between the metallic plates 12, 13, a part of the melted portion 26 may be scattered from a gap between the metallic plates 11, 12, and thus the spatter generation may be caused around the gap.
However, if the energization is stopped, the melted portion and hence the nugget are not grown to a sufficiently large size in the vicinity of the contact surface between the metallic plates 12, 13. Accordingly, a desired bonding strength is hardly achieved between the metallic plates 12, 13.
In Japanese Patent No. 3894545, the applicant has proposed that, in the process of spot welding such a stacked body, the pressing force of the first welding tip, applied to the low resistance workpiece, is made smaller than that of the second welding tip. In this case, the contact pressure of the low resistance workpiece against the high resistance workpiece is reduced. Therefore, the contact resistance between the low resistance workpiece and the high resistance workpiece is increased, so that a sufficient amount of Joule heating is generated at the contact surface. Consequently, the nugget between the low resistance workpiece and the high resistance workpiece can be grown to approximately the same size as the nugget between the high resistance workpieces, whereby the resultant stacked body can exhibit an excellent bonding strength.